Optical coupling

ABSTRACT

The invention relates to the coupling of light to and from an optical waveguide, such as an optical fibre. Light of a specific wavelength is deflected out from the fibre, or into the fibre, in a substantially transverse direction with respect to the propagation direction of light in the waveguide, by a deflector  16  arranged in the fibre core  10.  Wavelength selectivity of the deflector  16  is provided by a Bragg grating means located in the fibre core  10.  The deflected light is collimated, or converged towards a focus, by an interface  15  between a cladding  11,  having one index of refraction, and an outer medium  12,  having another index of refraction. The ratio between the radius of the cladding  11  and the radius of the core  10  made sufficiently small for the collimating, or converging, effect to appear.

TECHNICAL FIELD

[0001] The present invention relates to an optical coupling arrangementfor coupling light in to and out from an optical waveguide, preferablyan optical fibre.

TECHNICAL BACKGROUND

[0002] Light guiding in optical waveguides, and light guiding in opticalfibres in particular, is a well-known technology for transporting energyand information in the form of light. For example, as in the case ofoptical fibres, one-dimensional optical waveguides are based on lightguiding in a medium of cylindrical symmetry. The light guiding takesplace in a core, which is surrounded by a medium having a lowerrefractive index, the so-called cladding, light guiding according to asimple model being obtained by means of repeated total internalreflections between the core and the cladding. However, the light canonly propagate in certain predetermined directions, so-called modes,which are defined by certain phase conditions which must be met inconnection with the propagation of the light. According to the standardmodel, these modes consist of eigensolutions to Maxwell's equationsapplying existing cylindrical boundary conditions.

[0003] If the cross-sectional dimension of the core is sufficientlysmall, the light can only propagate in a single such mode. An opticalwaveguide with this characteristic is called an optical monomodewaveguide. Monomode waveguides have certain important advantages over awaveguide permitting several modes (multimode waveguide). For example,the information transfer capacity of an optical monomode fibre, oftencalled an optical single-mode fibre, is much greater than that of amultimode fibre when light is guided through a long fibre. Anotherimportant advantage of a monomode waveguide such as a single-mode fibreis its lack of ambiguity. Apart from the polarisation state of thelight, the characteristics of the light will be well-defined along theentire waveguide. In particular, the intensity distribution of the lightwill be well-defined along the entire waveguide. This is extremelyimportant in order to provide predictable operation of waveguide-basedcomponents.

[0004] Generally, several separate channels are utilised in order toincrease the transfer capacity of an optical waveguide, each channelconsisting of a specific light, wavelength. This technology is calledwavelength division multiplexing or WDM. In connection with WDM it isthus desirable to be able to add and subtract single wavelengthchannels, i.e. single light wavelengths, to and from the waveguide.

[0005] A well-known technology for wavelength-selective alteration ofthe propagation direction of light utilises optical phase gratings. Anoptical phase grating is a structure of essentially periodically varyingrefractive index in an optically transparent medium. When light isincident upon an optical phase grating a small part of the incidentlight is reflected by each grating element (period). When a plurality ofgrating elements are arranged in succession (i.e. arranged in a phasegrating) the total amount of reflected light will be the sum of all ofthese separate reflections. The part of the incident light that isreflected by each grating element depends on the depth (amplitude) ofthe refractive index modulation of the phase grating, i.e. on therefractive index difference of the grating elements. The greater themodulation the greater the part of the incident light that is reflectedby each phase element. If the propagation direction of the light whichis incident upon a phase grating is essentially perpendicular to thegrating, i.e. to the normal of the grating elements, the grating is saidto be operating in the Bragg domain and is called a Bragg grating. As aresult of the perpendicular incidence the light will be reflectedessentially parallel to the direction of incidence (i.e. in the oppositepropagation direction). The light which is reflected by each gratingelement will thus overlap the light reflected by all the other gratingelements, thus giving rise to interference. In a monomode waveguide, allreflections within a certain angle cone will couple to the only mode(propagation direction) permitted by the waveguide. In the case of thewavelength where these reflections are in phase, constructiveinterference arises, and despite the fact that each grating element onlyprovides a low intensity reflection, substantial reflection will beobtained for this wavelength from the grating as a whole. Thiswavelength, at which a substantial reflection is obtained from thegrating as a whole, is called the Bragg wavelength λ_(bragg) and isgiven (in connection with perpendicular incidence) by

λ_(bragg)=2nΛ

[0006] where n is the average value of the refractive index and Λ is theperiod of the phase grating. The reflectance for the Bragg wavelength isgiven by

R_(bragg)=tanh²κL

[0007] where L is the length of the Bragg grating in the propagationdirection of the light and κ is defined as$\kappa = \frac{4\quad \pi \quad \Delta \quad n}{\lambda}$

[0008] where Δn is the amplitude of the refractive index modulation.Since the refractive index modulation Δn typically is small (10⁻⁵-10⁻³),the above expression of the reflectance can be expanded into a powerseries, whereby it can be seen that the reflectance is approximatelyproportional to the square of Δn.

[0009] If the angle of incidence of the light upon the phase grating isnot perpendicular, i.e. if the grating planes are inclined, the lightwill not be reflected in the direction of incidence. By utilising aninclined phase grating, also known as a blazed grating, light can becoupled out from the core of the waveguide. Similarly, light can becoupled into the core of the waveguide by means of a blazed grating.

[0010] U.S. Pat. No. 5,042,897 (Meltz et al.) describes a device forcoupling light from a waveguide with the aid of tilted (blazed)gratings, i.e. phase gratings having grating elements (refractive indexvariations) whose planes intersect the propagation axis of the waveguideunder an angle which is different from 90 degrees. The angle at whichthe light will be coupled from the waveguide is determined by the angleof inclination of the grating elements in relation to the propagationaxis of the waveguide (the transverse phase matching condition) as wellas by the wavelength (the longitudinal phase matching condition). Thetilted grating elements function as small, almost completelytransparent, mirrors. The diameter of the mirrors (grating elements) isessentially equal to the diameter of the waveguiding structure. In asingle-mode fibre, for example, the waveguiding structure is composed ofthe core of the fibre, which usually has a diameter of about 10micrometers. Since this diameter is not much greater than the wavelengthof the light, the mirrors (grating elements) will cause diffraction ofthe reflected light. Consequently, the reflected light will spread outin a cone around the angle defined by the angle of inclination of thegrating elements. The transverse phase matching condition gives thatthis angle is about twice as large as the angle of inclination. Sincethe grating elements reflect light which is partially overlapping, acertain wavelength will only give rise to constructive interference ifthe light from each consecutive grating element is in phase with thelight from the preceding grating element. This occurs at a certainpredetermined angle, which is given (for a longitudinal grating) by thelongitudinal phase matching condition${\frac{2\quad \pi \quad N_{eff}}{\lambda} + {\frac{2\quad \pi \quad n_{clad}}{\lambda}\cos \quad \phi_{L}}} = {\frac{2\quad \pi}{\Lambda}\cos \quad \theta_{g}}$

[0011] where N_(eff) and n_(cladd) are the refractive indices of thewaveguiding structure (core) and the substrate (cladding) respectively,the substrate being assumed, in the above expression, to have aninfinite extension, φ_(L) being the output-coupling angle in thecladding, and θ_(g) being the angle of inclination.

[0012] Hence, light coupled out from a waveguiding core by means ofblazed gratings will exhibit significant divergence. Obviously, thisdivergence causes problems when the light is to be further processed.For example, it might be desirable to detect the light, modulate thelight or direct the light into another waveguiding core. Consequently,improvements are needed in order to overcome this problem of divergence.

[0013] It is to be understood that light beams, as described in thisapplication, are to be regarded as Gaussian beams. For example, thismeans that beams exhibit a waist when focused, and are diffractionlimited as regards divergence. The characteristic features of Gaussianbeams are well k)own to the man skilled in the art.

SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to eliminate, or atleast alleviate, the problems of divergence in connection withtransverse coupling of light to or from a waveguide, such as an opticalfibre. This object is achieved by an arrangement in accordance with theappended claims.

[0015] Typically, in a photosensitised fibre adapted for gratingwriting, the core has a diameter of a few micrometers (about 5 μm) andthe cladding has a diameter of more than 100 micrometers (about 125 μm).The difference in diameter between the core and the cladding is largeenough for light deflected out from the core to act as coming from apoint source (or a line source in the case of distributed deflection)with respect to the outer boundary of the cladding. Thus, lightdeflected transversally out from the core of the fibre will propagateradially out from the core. It is to be understood that, due todiffraction, the deflected light will spread out in a fan-like fashionfrom the core. Consequently, when light from the core reaches thecylindrical interface between the cladding and an outer medium, theangle of incidence is normal or very close to normal. Therefore, thelight will not be refracted to any noticeable extent, regardless of thedifference in retractive index between the cladding and the outermedium.

[0016] The present invention is based on the recognition that if thecladding of an optical fibre is provided with a small enough diameter,light deflected out from the core of the fibre will no longer act as apoint source (or line source) of light. Rather, if the cladding has adiameter that is sufficiently small compared to the diameter of thecore, light deflected out from the core will propagate as if coming froma distributed source. Hence, when the light reaches the interfacebetween the cladding and the outer medium, at least some of the lightwill be refracted, since the angle of incidence is greater than zero, atleast for some portion of the deflected light. According to theinvention, this effect is utilised in order to achieve a convergingeffect when light deflected from the fibre core passes the interfacebetween the cladding and the outer medium. Said converging effect can beutilised either for collimating the deflected light outside of thecladding, or for converging the deflected light towards a focus.

[0017] Thus, according to the present invention, an arrangementcomprising a waveguide, preferably an optical fibre, having a core and acladding is also provided with an outer medium interfacing the cladding.In the core, there is provided a deflector operative to deflect light into and out from said core. The refractive index of the outer medium ischosen 20 that it differs significantly from that of the cladding, andthe diameter of the cladding is such that light emanating from the corewill have an angle of incidence on the interface between the claddingand the outer medium greater than zero. Consequently, light passing theinterface between the cladding and the outer medium is refracted. By aproper choice of refractive indices of the cladding and the outermedium, and curvature of said interface there between (i.e. diameter ofthe cladding), light coupled out from a waveguiding core can be focuseda predetermined distance away from the interface. Similarly, light canbe coupled in to the waveguiding core in an advantageous manner byarranging a light source at or near said focus.

[0018] In one aspect, the present invention provides an optical couplingarrangement for coupling light transversally out from an opticalwaveguide, such as an optical fibre. The arrangement comprises adeflector provided in a core of the optical waveguide, said deflectorbeing operative to deflect light in said core in a substantiallytransverse direction out therefrom. Further, the arrangement comprises acladding interfacing the waveguiding core, said cladding having arefractive index which is lower than the refractive index of the core,in order to allow waveguiding in said core. The arrangement alsocomprises an outer medium interfacing said cladding, said outer mediumhaving a refractive index that substantially differs from the refractiveindex of the cladding. Furthermore, the curvature of the interfacebetween the cladding and the outer medium is such that light passingsaid interface is converged, and preferably focused at a predetermineddistance away from the waveguiding core. In a cylindrical symmetry, asin the case of an optical fibre, the refractive effect of the interfacebetween the cladding and the outer medium is achieved by the claddinghaving a diameter that is small enough, so that light emanating from thecore has an angle of incidence with respect to said interface which isgreater than zero. As described above, when the diameter of the claddingis sufficiently small, the core will no longer act as a point or linesource of light.

[0019] In order to allow wavelength selective coupling of light from thewaveguiding core, said core is preferably also provided with at leastone planar Bragg grating that establishes a resonance to a predeterminedwavelength. The planar grating is preferably superimposed upon a blazedphase grating constituting the deflector. Consequently, saidpredetermined wavelength exhibits an increased power density at saidresonance, whereby the deflection of the resonant wavelength out fromthe waveguiding core is enhanced. It is preferred that the planar Bragggrating is a chirped Bragg grating with a large amplitude of therefractive index modulation, thereby establishing resonances todifferent wavelengths in different portions thereof. Consequently,different wavelengths are coupled out from the core at differentpositions by the distributed deflector, i.e. the blazed phase grating.

[0020] In another aspect, the present invention provides an opticalcoupling arrangement for coupling light transversally into an opticalwaveguide, such as an optical fibre. The arrangement comprises adeflector provided in a core of the optical waveguide, said deflectorbeing operative to deflect light into said core from a direction that issubstantially transverse to the propagating direction of light in thewaveguiding core. The deflector has the feature of deflecting differentwavelength components at different positions along the same, thearrangement for coupling light thereby being wavelength selective. Thearrangement also comprises a cladding interfacing said core, and anouter medium interfacing said cladding. The refractive index of thecladding is lower than the refractive index of the core, in order toallow waveguiding in said core. The refractive index of the outer mediumdiffers substantially from the refractive index of the cladding, and thecladding has a sufficiently small diameter for the core to act as adistributed light source, light passing the interface between thecladding and the outer medium thereby being refracted, since the angleof incidence on the interface between the cladding and the outer mediumis greater than zero.

[0021] Preferably, the deflector is comprised of a distributedBragg-reflector that is tilted with respect to the propagation directionin the core of the waveguide, such distributed reflector also being knowas a blazed phase grating.

[0022] In one preferred embodiment of the present invention, an opticalcoupling arrangement comprises an optical fibre having a core and acladding, the cladding having a sufficiently small diameter in order forthe above-described converging effect to be noticeable. An outer mediumsurrounds the cladding of the fibre and has a refractive index thatdiffers substantially from the refractive index of the cladding, lightpassing the interface between the cladding and the outer medium therebybeing refracted due to the non-normal angle of incidence on saidinterface. In the core of the fibre there is provided a blazed phasegrating operative to deflect light propagating in said core outtherefrom in a substantially transverse direction. The core is furtherprovided with a chirped Bragg grating that establishes a plurality ofresonance regions, each resonance region being resonant to a differentwavelength component of the light propagating in the fibre core.Furthermore, the cladding of the fibre has a diameter such that lightdeflected from the core is focused, by the refracting power of theinterface between the cladding and the outer medium, at a predetermineddistance away from said core. By virtue of the presence of the chirpedgrating and the resonances established by the same, different wavelengthcomponents are deflected at different positions along said grating.Hence, different wavelength components deflected from the fibre core arebrought to a focus at different positions along the fibre. Thus, anarrangement according to the present invention provides transversewavelength selective coupling of light from an optical fibre, whereineach wavelength component is brought to a focus outside of the fibre.

[0023] It is to be understood that the propagation of light istime-invariant, meaning that the propagation direction can always beinverted. If light can propagate in one: direction along a path throughan optical system, light can also propagate in the opposite directionalong the same path. Consequently, wherever a propagation direction ismentioned in this specification, light can also propagate in theopposite direction. Specifically, this implies that wherever coupling oflight from a waveguiding core is described, the same arguments and factsapply also to coupling of light into a waveguiding core, whereapplicable.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In the following, a number of preferred embodiments of thepresent invention will be described in greater detail. The variousobjects and advantages of the invention will be more fully appreciatedwhen the detailed description is read in conjunction with theaccompanying drawings, on which;

[0025]FIGS. 1a and 1 b schematically show an embodiment of the presentinvention, in lateral and longitudinal cross-section, respectively;

[0026]FIGS. 2a and 2 b schematically show another embodiment of thepresent invention, in lateral and longitudinal cross-section,respectively;

[0027]FIG. 3 schematically shows an embodiment of the present invention,comprising two optical fibres between which light is coupled;

[0028]FIG. 4 schematically shows an embodiment of the present invention,comprising two optical fibres between which light is coupled and a lightmodulator arranged between said two fibres;

[0029]FIG. 5 schematically shows an embodiment of the present invention,in which light is coupled into an optical fibre from a light emittingdiode or a laser; or light emanating from the fibre core is detected bymeans of a photodiode;

[0030]FIG. 6 schematically shows an embodiment of the present invention,in which light coupled out from the waveguiding core, or light to becoupled in to the waveguiding core, is modulated by a light modulatorarranged at a location where light is brought to a focus;

[0031]FIG. 7 schematically shows an embodiment of the present inventionin which a plurality of optical coupling arrangements are provided incascade along the waveguiding core; and

[0032]FIG. 8 schematically shows another embodiment of the presentinvention in which a plurality of optical coupling arrangements areprovided in cascade along the waveguiding core.

[0033] In the figures, like parts are designated by like referencenumerals. Also, light rays are indicated by broken lines. Since thepropagation of light is time-invariant, the propagation direction is notindicated in the figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0034] In FIGS. 1a and 1 b, an arrangement is shown comprising a core10, a cladding 11 and an outer medium 12. In the core 10, there isprovided a deflector 16, comprised of a blazed phase grating. In thearrangement shown, light is deflected in one direction only from thefibre core 10. Said light is converged by the interface 15 between thecladding 11 and the-outer medium 12, and brought to a focus outside ofthe optical fibre (formed by the core 10 and the cladding 11). The shownsituation is very useful when launching light of a specific wavelengthinto the optical fibre, or when detecting light of a specific wavelengthdeflected out from the fibre. The shown arrangement is, in a sense, afundamental building block of arrangements according to the presentinvention. It can also advantageously be applied when modulating lightcoming from the fibre, or light to be coupled into the fibre. Thedifferent aspects of the arrangement will be further described below.

[0035] In order to achieve a converging effect at the interface 15between the cladding 11 and the outer medium 12, the diameter of thecladding 11 must be sufficiently small compared to the diameter of thecore 10. As mentioned above, a standard photosensitised fibre forgrating writing has a core with a diameter of about 5 μm and a claddingwith a diameter of about 125 μm. In such a fibre, the diameter of thecladding is so large, that light emanating from the core (i.e. that isdeflected out from the core in a substantially transverse direction) actessentially as a point or line source. Consequently, the angle ofincidence on said interface is normal or very close to normal, andrefraction of light at said interface is negligible. If, however, thediameter of the cladding is made sufficiently small, than light from thecore will act as coming from a distributed light source. Hence, at leastsome portion of said light will be incident upon said interface at anangle greater than zero and therefore be refracted. The refraction oflight at the interface does not come into force linearly when the ratiobetween the cladding diameter and the core diameter decreases. Rather,there is a distinct threshold interval where refraction at the interfacebecomes noticeable.

[0036] With a core diameter of about 4 μm, the cladding must have adiameter of about 45 μm or less for the converging effect to benoticeable. If the diameter of the core is about 6 μm, the diameter ofthe cladding must be about 100 μm or less before the converging effectis pronounced.

[0037] One embodiment of the present invention is shown in FIGS. 2a and2 b. Here, a core 10 and a cladding 11 of an optical fibre, as well asan outer medium 12, is enclosed by an external resonator. The refractiveindex of the fibre core 10 is higher than the refractive index of thefibre cladding 11, at least within a predetermined wavelength range, inorder to enable waveguiding in said core 10, The diameter of thecladding 11 is sufficiently small in order to provide non-normalincidence on the interface 15 between the cladding 11 and the outermedium 12. The refractive index of the outer medium 12 differssubstantially from the refractive index of the cladding 11, lightpassing the interface 15 between the cladding and the outer mediumthereby being refracted. Furthermore, the cladding 11 of the fibre isprovided with a diameter such that light passing the interface 15between the cladding 11 and the outer medium 12 is converged towards afocus outside the fibre, or at least collimated.

[0038] The external resonator enclosing the fibre is defined by a first13 and a second 14 mirror interfacing said outer medium 12. Preferably,the mirrors 13 and 14 are placed at or near a location where lightemanating from the fibre core 10 is brought to a focus by the convergingpower of the interface 15 between the cladding 11 and the outer medium12. In the core 10 of the fibre, there is provided a deflector 16comprised of a blazed phase grating. The blazed grating is operative todeflect light propagating in the core 10 in a substantially transversedirection out from the same. Consequently, light will be deflected bythe blazed grating into the external resonator, and the converging powerof said interface 15 will promote the stability of said resonator.Conveniently, light could be coupled out from or in to the fibre core 10by means of said external resonator.

[0039] Superimposed upon the blazed phase grating in the fibre core,there is provided a chirped Bragg grating. With a view to keeping thefigures intelligible, said chirped grating is not shown. The chirpedgrating has a modulation amplitude that is large enough to establishresonances to different wavelength components at different portionsthereof. Consequently, a specific wavelength component will exhibit anincreased power density at a specific portion of the fibre core, due tothe presence of said resonance. The increased power density at saidspecific portion will enhance the deflection of the correspondingwavelength. The deflecting power of the blazed grating can thus be madesmall enough to render deflection of non-resonant wavelength componentsnegligible. Hence, for example, different wavelength channels of awavelength division multiplexed optical signal propagating in the fibrecore will be deflected at different portions of the fibre core. Each ofthese channels is converged towards a focus outside the optical fibre bythe converging power of the interface between the fibre cladding and theouter medium and captured inside a respective external cavity.

[0040] With the geometry shown on the drawing, the refractive index ofthe outer medium 12 must be lower than the refractive index of thecladding 11 in order to achieve a converging effect at the interface 15.In other respects, the outer medium 12 can be comprised of any material,such as air, glass or plastic, as long as the above requirement on therefractive index is met.

[0041]FIG. 3 shows an embodiment of the present invention in which lightis coupled from a first fibre 41 to a second fibre 42. The first 41 andthe second 42 fibre comprise a respective core 101, 102 and cladding111, 112. Between said two fibres there is provided an outer medium 12,which interfaces both the first fibre cladding 111 and the second fibrecladding 112. The refractive index of the outer medium 12 differssubstantially from the refractive index of both the first fibre cladding111 and the second fibre cladding 112, light passing the interface 151,152 between the outer medium 12 and any of said claddings thereby beingrefracted. Preferably, the refractive index of the outer medium 12 islower than the refractive index of both claddings 111 and 112, lightpassing the interface between the outer medium and any of said claddingsthereby being converged.

[0042] In the core 101, 102 of each fibre, there is provided a deflector(not shown), preferably a blazed phase grating as described above,operative to deflect light out from and in to each core. The deflectoris wavelength selective, such that a specific wavelength is deflected ata specific region along the axis of the core. Hence, for example,different wavelength channels of a wavelength division multiplexedoptical signal are deflected at different portions along the core. Thewavelength selectivity is preferably obtained by a chirped Bragg that isbeing superimposed upon the deflector (the blazed phase grating). Theeffect of the chirped grating is described above.

[0043] The deflector in the first fibre 41 is operative to deflect lightout from the first fibre 41 in the direction of the second fibre 42. Thedeflected light is converged at the interface 151 between the firstfibre cladding 111 and the outer medium 12. When passing the interface152 between the outer medium 12 and the second fibre cladding 112, thelight is further converged towards a focus in the second fibre core 102.Light that is focused onto the second fibre core 102 is subsequentlydeflected into a guided mode in the second fibre core 102 by thedeflector provided in said core. Thus, an arrangement according to thepresent invention provides transverse coupling of light between twofibres, and the converging effect of the interface between a claddingand an outer medium augments the efficiency of the coupling.

[0044] In order to further increase the efficiency of the couplingbetween the two fibres, there is preferably provided two mirrors 131 and141 enclosing the fibres 41 and 42. Any light not coupled into a fibreby the blazed phase grating will continue towards one of said mirrors,and consequently be reflected back towards the fibre core again.Preferably, each mirror is provided at or near a location where lightfrom the fibre core is brought to a focus by the converging effect ofthe interface between the cladding and the outer medium. By symmetryconsiderations, it is obvious that light is focused onto the fibre coreafter reflection from a mirror arranged at a focus.

[0045] In FIG. 4, there is shown an arrangement much alike the one shownin FIG. 3. However, the arrangement shown in FIG. 4 further comprises alight modulator 155, which is provided in the outer medium 12 betweenthe first 41 and the second 42 fibre. The light modulator 155 can beutilised for damping and/or modulating the light coupled between saidtwo fibres. In particular, since light of only a specific wavelength iscoupled between the fibres at a specific portion thereof, the lightmodulator can advantageously be utilised for modulating a single channelwithin a wavelength division multiplexed optical signal.

[0046]FIG. 5 shows schematically an arrangement according to the presentinvention, in which light of a specific wavelength is launched into anoptical fibre 40; or in which light of a specific wavelength is coupledout from the fibre and detected by means of a photodetector. Thus, theschematic symbol of a diode (61) in the figure can designate either aphotodetector, or a light source such as a light emitting diode or alaser diode of course, any other type of light source is alsoconceivable.

[0047] In a first example, illustrated by FIG. 5, light is coupled outfrom the fibre 40 and detected by a photodetector 61. In the core 10 ofthe fibre 40, there is provided a deflector (not shown), preferablycomprised of a blazed phase grating as described above, which isoperative to deflect light of a specific wavelength towards thephotodetector 61. The wavelength selectivity is preferably obtained bymeans of a chirped Bragg grating superimposed upon the deflector, asdescribed above. Light deflected out from the fibre core is converged atthe interface 15 between the fibre cladding 11 and the outer medium 12towards a focus at the photodetector 61. In order to collect any lightpossibly deflected out from the fibre 40 in the opposite direction, i.e.away from the photodetector 61, there is preferably provided a mirror 62at or near a location where light in the opposite direction is broughtto a focus by the converging power of the interface 15 between thecladding 11 and the outer medium 12. Thus, light is reflected by saidmirror 62, and Subsequently arrives at the photodetector 61.

[0048] In a second example, also illustrated by FIG. 5, light islaunched into the fibre from an external light source 61. The lightsource is preferably a laser diode, although any other light source isconceivable within the scope of the invention. The light source 61 isarranged at or near a location where light emanating from the fibre core10 would be brought to a focus by the converging power of the interface15 between the cladding 11 and the outer medium 12. Consequently, lightfrom the light source,61 is brought to a focus at the fibre core 10, anddeflected into a guided mode in the fibre core by the deflector providedtherein. As before, it is preferred that the deflector is comprised of ablazed phase grating. On the opposite side of the fibre, with respect tothe light source 61, there is provided a mirror 62. Any light passingthrough the fibre core 10 without being deflected into a guiding modetherein is thus reflected back towards the core 10 by the mirror 62. Themirror 62 is placed at or near a location where light is brought to afocus by the converging effect of the interface 15 between the cladding11 and the outer medium 12. In some cases, it might be desirable tolaunch light into the fibre from one side only, in which case the mirror62 is simply removed or left out. In order to enhance the coupling oflight to the fibre, a chirped Bragg grating is superimposed upon theblazed phase grating in the fibre core. As described above, the chirpedgrating provides regions of increased power density for differentwavelength components at different portions along the fibre. By virtuethereof, the coupling of a specific wavelength at the region where saidwavelength is resonant, is enhanced.

[0049] Instead of arranging a light source or a photodetector at or nearthe focus, as schematically shown in FIG. 5, a light modulator 155 couldbe arranged at said focus. Such an arrangement is schematically shown inFIG. 6. The light modulator 155 can be utilised for damping ormodulating light emanating from the fibre or light to be launched intothe fibre. Remember that the coupling of light to and from the fibre bythe arrangement according to the present invention is wavelengthspecific. Thus, a modulator could be operative to modulate a carrierwavelength before it is launched into the fibre. The carrier wavelengthis then provided by a transmitter of any convenient kind (not shown).

[0050] On the other hand, the carrier wavelength could also be extractedfrom the fibre. In such a case, the fibre carries a continuous wavesignal, comprising a plurality of wavelength channels. Each channel isdeflected out from the fibre at a different location, by virtue of achirped Bragg grating as described earlier, and focused by the interfacebetween the cladding and the outer medium towards a respective modulator155, where it is modulated with an information signal. This situation isclosely related to the arrangement schematically shown in FIG. 4.Advantageously, the first fibre shown in FIG. 4 could carry a continuouswave carrier comprising a plurality of wavelength channels. Each carrierchannel is deflected at a different portion along the first fibre, andconverged by the interface between the first fibre cladding and theouter medium towards the light modulator. At the modulator, the carrieris modulated with an information signal on the fly. Subsequently, themodulated carrier is converged at the interface between the outer mediumand the second fibre cladding towards the second fibre core, in which itis deflected into a guided mode, as described earlier.

[0051] The cascadeable feature of the present invention is illustratedin FIGS. 7 and 8. In FIG. 7, a continuous blazed grating 16 is utilisedfor the deflection of light in to and out from the fibre core 10. Anexternal resonator (defined by mirrors 13 and 14) is provided, thatencloses the fibre core 10, the fibre cladding 11 and the outer medium12. In the case shown in FIG. 7, the mirrors in different couplingregions are contiguous in the sense that no physical separations betweenthe mirrors 13 and 14 on any one side of the fibre exist. When utilisinga continuous deflecting grating 16, as shown in FIG. 7, it is preferredthat the wavelength selective grating (which is superimposed upon thedeflecting grating) is a chirped Bragg grating.

[0052] As shown in FIG. 8, it is also possible to have a dedicateddeflecting grating 16 a, 16 b, 16 c and a dedicated pair of externalmirrors 13 a, 14 a, 13 b, 14 b, 13 c, 14 c for each coupling region. Inthis case, the wavelength selectivity can still be provided by means ofa chirped Bragg grating extending through all coupling regions. However,it is also possible to have a separate wavelength discriminating gratingin each coupling region, each grating then being superimposed upon therespective deflecting grating 16 a, 16 b, 16 c, or provided on bothsides of the respective deflecting grating in, the fibre core.

[0053] The invention has been described above with reference toembodiments in which light emanating from a fibre core is convergedtowards a focus by the refracting effect of an interface between acladding and an outer medium. Or, equivalently, light coming from theouter medium is converged by the refracting effect of the interfacetowards a focus in the fibre core. However, it is also conceivablewithin the scope of the intention to tailor the curvature of saidinterface between the cladding and the outer medium in order to obtainany shape of the wavefront after the light has passed the interface.This could be done by making the interface between the cladding and theouter medium non-cylindrical, or cylindrical with a superstructure onthe surface of the cladding.

1. An optical arrangement, comprising a waveguiding core having a firstradius and a first index of refraction; a cladding surrounding saidcore, the cladding having a second radius and a second index ofrefraction that is lower than said first index of refraction, at leastfor a predetermined wavelength range; an outer medium interfacing saidcladding, the outer medium having a third index of refraction that islower than said second index of refraction, at least for saidpredetermined wavelength range; and a deflector provided in saidwaveguiding core, the deflector being operative to deflect light outfrom said core in a substantially transverse direction, wherein theratio between said second radius and said first radius is such thatlight emanating from the core is collimated, or converged towards afocus, by the interface between the cladding and the outer medium.
 2. Anarrangement as claimed in claim 1, wherein the deflector comprises ablazed phase grating.
 3. An arrangement as claimed in claim 1 or 2,further comprising a chirped Bragg grating provided in the waveguidingcore, said chirped grating being superimposed on the deflector.
 4. Anarrangement as claimed in any one of the preceding claims, furthercomprising a first mirror interfacing the outer medium and beingarranged to reflect light incident thereupon.
 5. An arrangement asclaimed in claim 4, wherein the first mirror is arranged to reflectlight emanating from the waveguiding core back towards said waveguidingcore.
 6. An arrangement as claimed in claim 5, wherein the mirror isprovided at a location where light emanating from the waveguiding coreis brought to a focus by the interface between the cladding and theouter medium.
 7. An arrangement as claimed in claim 4, furthercomprising a second mirror interfacing the outer medium, the secondmirror being arranged in a way such that the first mirror and the secondmirror defines a resonator enclosing the waveguiding core, the claddingand the outer medium.
 8. An arrangement as claimed in any one of thepreceding claims, further comprising a detector provided at or near apoint where light from the waveguiding core is brought to a focus by theinterface between the cladding and the outer medium, said detector beingarranged to detect light deflected out from the waveguiding core.
 9. Anarrangement as claimed in any one of the preceding claims, furthercomprising a light source provided at or near a point where lightemanating from the core is brought to a focus by the interface betweenthe cladding and the outer medium, said light source being arranged tolaunch light into the waveguiding core, light being deflected into aguided mode in said core by the deflector.
 10. An arrangement as claimedin any one of the preceding claims, further comprising an additionalwaveguiding core and an additional cladding surrounding said additionalcore; the outer medium interfacing both the first-mentioned cladding andthe additional cladding, and said additional core being provided at alocation where light emanating from said first-mentioned core exhibits afocus.
 11. An arrangement as claimed in claim 10, further comprising anexternal resonator enclosing both waveguiding cores and both claddings,which resonator is defined by a first and a second mirror.
 12. Anarrangement as claimed in claim 10 or 11, wherein the propagationdirection of light in the second waveguiding structure is substantiallyparallel to the propagation direction of light in the waveguiding corefrom which light is emanating, said second waveguiding structurecomprising a deflector operative to deflect light which is incidenttransversally thereupon into a guided mode in said second waveguidingstructure,
 13. An arrangement as claimed in any one of the precedingclaims, wherein the outer medium is comprised of air.
 14. An arrangementas claimed in any one of the claims 1 to 12, wherein the outer medium iscomprised of glass.
 15. An arrangement as claimed in any one of theclaims 1 to 12, wherein the outer medium is comprised of plastic.
 16. Anarrangement as claimed in any one of the preceding claims, comprising aplurality of coupling regions arranged in cascade along the waveguidingcore, each of said coupling regions being operative to couple a specificwavelength in to and out from said core.
 17. An optical arrangement,comprising a waveguiding core having a first index of refraction; acladding surrounding said core, the cladding having a second index ofrefraction that is lower than said first index of refraction, at leastfor a predetermined wavelength range; an outer medium surrounding saidcladding, the outer medium having a third index of refraction that islower than said second index of refraction, at least for saidpredetermined wavelength range; and a deflector provided in saidwaveguiding core, the deflector-being operative to deflect light outfrom said core in a substantially transverse direction, wherein thecurvature of the interface between said cladding and said outer mediumis such that light emanating from the core and passing said interface isrefracted and given a wavefront having a desired shape in accordance iswith said curvature.